The present invention relates to a DC/DC converter, and more particularly, to a synchronous rectification type DC/DC converter, a semiconductor device functioning as a DC/DC converter, and an electronic device and a battery pack, which incorporate a DC/DC converter.
A portable electronic device, such as a notebook personal computer, a personal digital assistant (PDA), and a cellular phone handset, is provided with a battery, which functions as a power source. Further, a portable electronic device incorporates a DC/DC converter for charging the battery with power supplied from an external power source, such as an AC adapter. The DC/DC converter alternately activates a main transistor and a synchronization transistor, which are connected in series, to supply a load mounted in the electronic device with constant voltage. These electronic devices now have increased performance even though they have become more compact in size. Thus, there is a demand for a more compact DC/DC converter.
FIG. 1 shows an example of a synchronous rectification type DC/DC converter 1 in the prior art. The DC/DC converter 1 includes a control circuit 2, a main transistor T1, and a synchronization transistor T2, which are all configured on the same chip of a semiconductor integrated circuit substrate. The main and synchronization transistors T1 and T2 are N-type MOS-FETs controlled by the control circuit 2.
The control circuit 2 provides the gate of the main transistor T1 with a first drive signal SG1 and the gate of the synchronization transistor T2 with a second drive signal SG2. The main transistor T1 functions as a main switch that drives a load mounted on the electronic device. The drain of the transistor T1 is supplied with voltage Vi from an AC adapter (not shown). The source of the transistor T1 is connected to the drain of the synchronization transistor T2.
The source of the synchronization transistor T2 is connected to ground GND.
Further, the source of the main transistor T1 is connected to an output terminal 3 via a choke coil L1 and a resistor R1, which configure a smoothing circuit. The source of the main transistor T1 is connected to the cathode of a flyback diode D1. The anode of the flyback diode D1 is connected to ground GND.
The node between the choke coil L1 and the resistor R1 is connected to ground GND via a smoothing capacitor C1, which configures a smoothing circuit. The output terminal 3 is connected to a battery BT, which is attached to the electronic device. The output terminal 3 is connected to an internal circuit (not shown), which is configured by a central processing unit (CPU) installed in the electronic device. The DC/DC converter 1 outputs an output voltage Vo from the output terminal 3. Resistors R2 and R3 divide the output voltage Vo to generate divisional voltage V2 and return the divisional voltage V2 to the control circuit 2.
The control circuit 2 includes an error amplification circuit 11, a PWM comparison circuit 12, a triangular wave oscillation circuit 13, a pause period setting circuit 14, a first output circuit 15, a second output circuit 16, and a regulator 17. The control circuit 2 sets the pulse widths of the first and second drive signals SG1 and SG2 in accordance with the difference between the divisional voltage V2 and the reference voltage Vr. Then, in the control circuit 2, the first output circuit 15 provides the main transistor T1 with the first drive signal SG1, and the second output circuit 16 provides the synchronization transistor T2 with the second drive signal SG2. As a result, the control circuit 2 alternately activates the main transistor T1 and the synchronization transistor T2 with predetermined frequencies. In this manner, the DC/DC converter 1 controls the main transistor T1 and the synchronization transistor T2 so that the output voltage Vo is maintained at a constant level.
In the DC-DC converter 1, a bootstrap decreases the ON resistance of the main transistor T1 and improves the energy converting efficiency.
More specifically, the DC/DC converter 1 includes a boot capacitor C2. The boot capacitor C2 is connected between a node N1, of the main transistor T1 and the synchronization transistor T2, and the cathode of a diode D2. The anode of the diode D2 is connected to an output terminal of a regulator 17, which is arranged in the control circuit 2. Further, the anode of the diode D2 is connected to the ground via a capacitor C3. The cathode of the diode D2 is connected to a power supply terminal of the first output circuit 15. The regulator 17 generates regulator output voltage Vb from the input voltage Vi and supplies the regulator output voltage Vb to the second output circuit 16.
If the main transistor T1 is inactivated when the synchronization transistor T2 is activated, the main transistor T1 has a source potential that is the ground potential. In this state, current flows from the regulator 17 to the capacitor C2 via the diode D2. Thus, the capacitor C2 is charged until the voltage of the capacitor C2 is equalized with the regulator output voltage Vb. The first output circuit 15 then uses the charge voltage of the capacitor C2 to provide the gate of the main transistor T1 with the drive signal SG1. This activates the main transistor T.
When the main transistor T1 is activated, the source potential of the transistor T1 increases to the input voltage Vi. In this state, the capacitor C2 is connected to the source of the transistor T1. Thus, voltage Vs supplied to the first output circuit 15 from the capacitor C2 increases until it becomes higher than the input voltage Vi (Vs=Vi+Vb).
Accordingly, the first output circuit 15 uses the voltage (Vi+Vb), which has been increased by the bootstrap, to provide the gate of the main transistor T1 with the drive signal SG1. In this manner, the increased voltage (Vi+Vb) activates the main transistor T1. Thus, the ON resistance of the main transistor T1 is relatively small.